Not sure what heat sink compound looks like ... never actually gotten around to buying some or seen someone using it. (It's been on my 'to-buy' list for a long time, for general shop supplies ... just never gotten around to it ... yet.)
Looking at your picture, I'd say it's probably heat sink compound.
The sealant on the back of the module would be a thicker, gooier, solidified mess if that had melted. (I've seen pictures online of someone melting and removing the sealant ... to do something I wouldn't recommend.)
Oh, don't try to clean up the appearance of your module ... paint and a lot of other 'appearance improvements' would just make heat issues worse. (I might be able to very carefully do an appearance restoration. But you'd have to send the module to me and I'd have to charge for the service. (Doing so is one of the businesses I run.)
